Large scale screening of major components of society is a desirable public health tool for the initial detection of possible common disease conditions. For example, periodic testing of the visual system of students for the early detection of glaucoma and other diseases is desirable because early treatment can minimize the extent of damage to the system.
It is desirable that equipment for performing such testing have the capability of being used easily, of performing the test quickly and comfortably, and of providing reliable test results including as much information as possible about a suspected disease in the event of a positive test result.
Many sensory systems of vertebrates, including humans, perform two types of tasks. One type of task is the detection of amplitude or detail information requiring an essentially linear relationship between the input stimulus and the output to the brain. The other, typically involving pattern recognition, requires a strongly nonlinear relationship. Generally, these two types of sensory tasks are carried out by distinct sensory pathways which differ in their susceptibility to various diseases.
For example, the human visual system is composed of two distinct pathways commonly referred to as the X or linear pathway and the Y or nonlinear pathway. The linear pathway, which is responsible for color and acuity vision, originates predominately in the central macula of the human retina. The nonlinear pathway, which detects spatial and temporal patterns, predominates in peripheral vision.
The linear visual pathway originates in small diameter retinal ganglion cells which are highly susceptible to impaired retinal circulation, such as caused by diabetes. Thus, it would be desirable to determine from a test of the human visual system whether there has been degeneration of the linear pathway.
The nonlinear visual pathway originates in large diameter retinal ganglion cells which are highly susceptible to increased intraocular pressure such as is caused by glaucoma. Thus, it would be desirable to detect degeneration of the nonlinear pathway as a result of such a disease condition.
In the past, prior workers have flashed a spot of light on the peripheral retina and also upon the macula, measured transient electrical responses evoked on the scalp and compared those responses. However, this is unsatisfactory because the nonlinear component of the visual response cannot be determined from such measurements. Furthermore, the amplitude measurements are essentially useless for comparing to the amplitude measurements derived from other patients because they are dependent upon the instrumentation itself and the manner in which it is attached to the patient. For example, the amplitude measurements are dependent upon the amount of retinal stimulation and the resistance between the skin and the electrode conventionally used for detecting evoked potentials. Thus, such a system is of little or no value when detecting whether a particular individual suffers from a disease.
Prior workers have applied visual stimulation to the human eye and detected the evoked response. Such systems are disclosed in U.S. Pat. Nos. 3,087,487, 3,172,404, 4,181,407 and 4,493,539. Other patents which test vision or examine evoked responses are U.S. Pat. Nos. 4,293,200, 4,493,327, and 4,462,411.
All of these systems suffer from two major difficulties. First, the test results of each system are also dependent upon (or a function of) the strength of the input visual stimulation signal and the electrical characteristics of the monitoring and recording apparatus. Secondly, these systems are unable to distinguish disease affecting the linear system from disease affecting the nonlinear system when screening a number of different individuals.
It is therefore an object and feature of the present invention to provide a method for testing a neurological sensory system of a vertebrate for disease conditions in a manner which provides normalized test results which are essentially independent of the input signal strengths, anatomical characteristics not being tested, the test equipment and the character of its attachment to the patient being tested.
It is another object and feature of the present invention to provide such a testing system which not only can detect the likelihood of the presence of disease, but additionally can signal the extent of the disease and whether the disease has caused degeneration of the linear pathway or the nonlinear pathway, thus permitting the suspected disease condition to be signalled.